CDM Smith has used SWMM extensively for rural areas in Florida (e.g., Tomoka River, Orlando Lake Hart, Big Slough, North
Fork St Lucie River, Anclote River, Lower St Johns River tributaries, Munson
Slough, and several water control district agricultural canal systems). The
following descriptions come from our University of Florida TREEO SWMM Class
(CDM Smith, 1991 - 2002). Here is some guidance in the application:

1) Two runoff planes are simulated for each catchment (basin or hydrologic unit):

the impervious runoff plane based on the directly connected impervious area (DCIA) percentage to represent existing wetlands and water, and

the pervious (and NDCIA) runoff plane (which can be
modeled using Horton or Green-Ampt methods). Often NDCIA is a small percentage
of the total and can be neglected or accounted for in the weighted soils
parameters.

Here are some typical ranges of Horton parameters based on multiple gage calibration:

This soil storage S can be related to Curve Number (CN) if desired using the
standard SCS equation S= (1000/CN)-10.

Defined
Antecedent Moisture Conditions (AMCs) for Pervious Areas

Condition

Description

Total
rainfall during 5 days preceding storm
(inches)

1

Dry

<1.4

2

Normal

1.4 -
2.1

3

Wet

>2.1

4

Saturated

Saturation

2) Overland flow paths
should be estimated using two to three alternate paths
and weighted for representative area (fraction of the catchment), overland flow
slope, and Manning n roughness. The overland flow paths should reflect the
average area-weighted path (which is not the hydraulically most distant path for
the entire catchment); hence the multiple paths to reflect the average area
weighted value.

Manning n roughness values can vary from 0.1 to 0.6 and
higher (pine palmetto flatwoods or other rough surface) since the depth of
overland flow are often much less than the roughness feature height.

We have used overland flows paths from 100 to 3,000 feet
depending on catchment size. Width is calculated from the digitized areas and
the estimated weighted overland flow length using the equation: A= L*W. The
resultant width should be checked for general physical equivalence after the
calculation and the overland flow paths refined if necessary.

Here is an example table for Manning n for overland flow:

Factors
Used For Calculating The Standard Infiltration Curves For Pervious Areas

4) Sensitivity analyses should be considered for parameters which are less
known (e.g., slope based on USGS 5 foot contour maps or soils storage based on
soils type and depth to water table). Previous sensitivity analyses have
indicated that for a given rainfall event and area, the most dominant parameters
are DCIA, and then soils parameters and slope (although slopes in Florida are
generally moderate to flat).